The present invention relates generally to signal injectors, and more particularly, to an active distributed signal injector for use with high frequency systems, whose components are interconnected by low loss constant impedance transmission lines.
In many electronic systems, the need arises to inject an auxiliary signal into an existing signal path, while minimizing attenuation, distortion, or other perturbation of the through signal. Some examples of such systems include interference or distortion cancellation circuits, interference suppressors, feedforward amplifiers, and equipment for testing or monitoring, electronic components and systems.
One prior art signal injection circuit uses a low-loss passive coupler. A loose coupling, typically -20 dB, is used to minimize degradation to the main signal caused by attenuation or distortion in phase and amplitude response. Unfortunately the coupling loss suffered by the injected signal has to be made up by an additional amplifier, most of whose output is wasted in the termination at the output port. Thus, a compromise has to be made between loss to the injected signal and the main signal, but attenuation to either requires additional amplification, which adds its own penalty in signal degradation, power, cost, weight, and space.
Another prior art signal injection circuit uses a high impedance amplifying device, such as a field effect transistor (FET). This circuit avoids the penalty of wasting power in a termination, but, since the circuit has no directivity, half the power is wasted in the source impedance of the main input. This unwanted signal component, which travels in the wrong direction, can also cause other difficulties, such as harmful feedback and reflections. Also, lumping all the amplifying function in one large device, makes impedance matching difficult.
Distributed amplifiers are similar to the present invention. Distributed amplifiers are disclosed in a number of U.S. patents. For example, U.S. Pat. No. 4,359,695 entitled "Electronic amplification systems" discloses a distributed amplifier system allowing easier correction of amplifier errors. This invention provides an electronic amplification system incorporating a distributed amplifier, which system comprises comparison means for comparing the shape of the input control signal with the shape of the final output signal, and for providing a difference signal indicative of any amplifier-induced distortion, correction signal generating means for forming in dependence upon the difference signal a correction signal, and combiner means for combining the correction signal with the final output signal so as substantially to cancel the amplifier-induced distortion.
U.S. Pat. No. 4,845,440 entitled "Distributed amplifier circuits" discloses a wide band amplifier for operation at very high frequencies that comprises a MESFET distributed amplifier having a gate and a drain transmission line, a first hybrid circuit to apply a first and second input signal to opposite ends of the gate transmission line, and a second hybrid circuit connected to opposite ends of the drain transmission line to receive and combine first and second output signals from the drain transmission line to provide an amplified output signal. The use of two input signals traveling in opposite directions along the gate transmission line increases the gain which can be achieved in the distributed amplifier and reduces the noise component of the output signal. This circuit is useful for enhancing the performance of a distributed amplifier containing only a few MESFETs.
U.S. Pat. No. 5,196,805 entitled "Distributed differential amplifier arrangement discloses an N-stage differential distributed amplifier arrangement. The differential distributed amplifier arrangement includes a parallel connection of N-differential amplifiers. The inputs to the amplifiers are delayed so that the same input is received by each amplifier in sequence at a slightly later time than the preceding amplifier. The outputs of each amplifier are also delayed so that the output of the previous amplifier is added to the output of the next sequential amplifier. Thus, the output is an amplified version of the input. By appropriate grounding of inputs or outputs the differential distributed amplifier arrangement may convert from balanced signals to single-ended signals, from single-ended signals to balanced signals or from two inputs to two outputs.
U.S. Pat. No. 4,797,628 entitled "Distributed push-pull amplifier" discloses a modified distributed amplifier that is capable of providing push-pull operation without the loading losses of conventional push-pull combining. The modified distributed amplifier comprises a distributed amplifier configuration and with signal inverting means, such as a wide bandwidth transmission line transformer, interconnected into both the input and output lines. The signal inverting means are placed at the electrical centers of the lines, but may be placed at any positions in the individual lines to produce optimum performance to specific applications. The separate segments of the distributed amplifier separated by the signal inverting means operate in opposed phase but the signals output to the load add in phase thus providing push-pull operation. Since only one reverse terminating resistor is required, the power normally lost due to the loading by the companion amplifier of a conventional push-pull combined distributed amplifier system is instead delivered to the output load, the use of this type of amplifier preserves the advantages of the distributed amplifier configuration while providing the superior performance of the push-pull configuration but without the losses normally incurred with conventional push-pull combining.
U.S. Pat. No. 5,166,640 entitled "Two dimensional distributed amplifier having multiple phase shifted outputs" discloses a two dimensional distributed amplifier phase shifter having a distributed reference amplifier circuit for generating a reference signal. The reference amplifier circuit has its input connected to one end of a plurality of serially connected microstrip transmission lines. The phase shifter has a plurality of phase shifted amplifier circuits, one associated with each of the microstrip transmission lines. Each phase shifted amplifier circuit has an input connected to an end of its associated microstrip transmission line which is opposite the reference amplifier circuit and generates an output signal, phase shifted from the reference signal or the output signal of an adjacent phase shifted amplifier by a predetermined phase angle.
U.S. Pat. No. 5,177,381 entitled "Distributed logarithmic amplifier and method" discloses a logarithmic amplifier with amplifier stages having an input transmission line and first and second output transmission lines. The input and output transmission lines are coupled by multiple amplifier elements distributed along the transmission lines. One output transmission line forms a high gain low compression path and the other a low gain high compression path. The output transmission lines of each stage are coupled to a combiner from whence the logarithmically amplified output signal is obtained. The logarithmic amplifier stages are readily constructed in MMIC form and multiple stages may be easily cascaded to provide a very large dynamic range.
U.S. Pat. No. 4,752,746 entitled "Wideband microwave matrix amplifier" discloses a microwave amplifier that multiplicatively and additively amplifies microwave frequency signals. The matrix amplifier is a distributed amplifier with two or more tiers (rows) of transistors. Each tier has a plurality of transistors that additively amplify the signal entering that row of the amplifier, and each row multiplicatively amplifies the output of the previous row. The gates of the transistors in each row are sequentially coupled to an input transmission line having a series of transmission elements. The outputs of all the transistors from each row are sequentially coupled to the input transmission line of the next tier, except that the outputs of the last tier are coupled to an output transmission line for transmitting the output of the amplifier to an output node. Each transmission lines has at least one line termination at one of its ends for absorbing signals incident on that end of the transmission line, and biasing means for DC biasing the transmission line at a corresponding voltage potential.
U.S. Pat. No. 5,070,304 entitled "Distributed combiner power amplifier and method" discloses a distributed amplifier usable in the gigahertz frequency range for radar and electronic warfare applications. In the amplifier stage, a combination of optimum amplifier stage loading, a reflection canceling output network, and an amplifier capacitance hiding input arrangement are employed.
Other U.S. patents relate to feedforward amplifiers. These include, for example, U.S. Pat. No. 4,359,695 entitled "Electronic amplification systems" cited above, and U.S. Pat. No. 4,359,696 entitled "Amplifiers" which discloses a feedforward amplifier that compensates for distortions introduced by a main amplifying device by comparing the amplified signal with the original unamplified signal and producing a correction signal, which is related to the difference. The correction signal is combined with the main amplified signal at a combiner. The combiner takes the form of a tetrode or pentode valve which acts as a current source and which is designed so as to constitute part of an output transmission line having a predetermined characteristic impedance. By this means the combiner does not adversely affect the main amplified signal and the capacitance inevitably associated with a tetrode or pentode valve is utilized as part of the reactance of the transmission line and does not adversely affect the frequency response of the amplifier.
U.S. Pat. No. 4,885,551 entitled "Feed forward linear amplifier" discloses a feed forward circuit receives an input signal having at least one carrier therein in a prescribed frequency range. The input signal is applied to a first circuit path having a power amplifier that produces an output signal with a distortion component and to a second circuit path that delays the input signal without distortion. The output signal of the first circuit path is combined with the output signal of the second circuit path to form a signal representative of the distortion component of the first circuit path output signal and the distortion component representative signal is subtracted from the output signal of the first circuit path to cancel the distortion component therein. The prescribed frequency range of the first circuit path output is scanned to detect a carrier signal and the amplitude and phase of the signal in the first circuit path is modified to minimize the magnitude of the detected carrier in the distortion representative signal.
While these last-cited U.S. patents discuss feedforward amplifiers, none feature distributed amplifiers with two input signals, one of which is amplified using active devices, while the other is passed through without active amplification. Active amplification is prone to introduce distortion. The present invention could form part of a feedforward amplifier, but it is not in itself a feedforward amplifier.
Accordingly, it is an objective of the present invention to provide an active distributed signal injector for use with high frequency systems, whose components are interconnected by low loss constant impedance transmission lines.